Electron transfer to O₂ is a universally existing process for the physiochemistry of many materials. Electron transfer to O₂ is also an inevitable process for photocatalytic reactions over TiO₂ and other materials. In the present research, a diffusion reflectance system was developed to measure in situ optical diffusion reflectances caused by photoinduced electrons in nano-TiO₂ under a steady light illumination; in situ absorption decays can be obtained to study the electron transfer from their trapped states to O₂. It is seen that the kinetics of electron transfer to O₂ is persistent and dispersive; this lasts for several minutes and approximately agrees with a stretched exponential kinetics. The result implies that variable apparent energy barriers (E_is) are involved in the electron transfer. The effects of O₂ amount, light intensity, and temperature are studied and the results mean the trap-filling effect should be involved in the electron transfer to O₂. A Laplace transform is used to derive the E_i distributions. It is found that the E_i dispersion shape almost does not change; this indicates that the physical reason causing the E_i dispersion is the same for different experimental conditions and possibly comes from the trap-filling effect. It is shown that the slow kinetics of the electron transfer is also dependent on the slow rate for an electron transferring from a trap to O₂, in additional to the trapping-filling effect. The results indicate that the photocatalytic activity can be increased through a modulation in trap distribution.